HARQ adaptation for acquisition of neighbor cell system information

ABSTRACT

A method in a mobile station including receiving an order from a serving cell for system information acquisition of a neighbor cell, wherein the order includes at least a physical cell identifier and a time limit for acquisition of the system information of the neighbor cell, acquiring the system information of the neighbor cell within the time limit for the acquisition of the system information, and reporting at least a part of the acquired system information to the serving cell.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 12/792,194, filed Jun. 2, 2010, which claims benefit under 35 USC119(e) to U.S. Provisional Application No. 61/219547 filed on Jun. 23,2009, the disclosures of which are hereby incorporated by referenceherein.

FIELD OF DISCLOSURE

The present disclosure relates generally to wireless communications andmore specifically to acquisition of system information of neighboringcells.

BACKGROUND

The reading of neighbor cell system information by a mobile station in awireless communication network is beneficial in many situations. Onesuch situation arises in 3^(rd) Generation Partnership Project (3GPP)Long Term Evolution (LTE) networks in the presence of Home eNBs (HeNBs),which are also referred to as femto cells. An LTE HeNB is a low powereNB that provides coverage in hotspots, e.g., in homes, etc., to a smallnumber of users. A physical cell identifier (PCID) uniquely identifieseach macro cell, since the network is planned and deployed such thatmacro cells with the same PCID are separated by a very large distance tofacilitate re-use of PCIDs. In contrast, HeNBs are deployed by users inan uncoordinated fashion with no PCID planning. As a result it ispossible to have two HeNBs under the coverage of a macro cell that usethe same PCID, which may result in PCID confusion.

For handover, the network needs to know precisely to which cell tohandover a mobile station. One approach to resolve PCID confusion is forthe mobile station to read the unique cell identifier (cell global ID orCGI) or CSG ID in System Information block 1 of the target HeNB and toreport the CGI or the CSG ID. However, mobile stations in connected modein LTE Rel-8 are not required to acquire system information messagesfrom neighboring cells. Reading neighbor cell system information cancause significant interruptions to an ongoing connection with a servingcell. This interruption is particularly problematic when there is anongoing voice call. Thus it is desirable to reduce loss of data packets(e.g., voice frames) transmitted between a mobile station and a servingcell when the mobile station attempts to acquire neighbor cell systeminformation.

The various aspects, features and advantages of the disclosure willbecome more fully apparent to those having ordinary skill in the artupon careful consideration of the following Detailed Description thereofwith the accompanying drawings described below. The drawings may havebeen simplified for clarity and are not necessarily drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a method for receiving system information transmittedby a neighbor cell by shifting HARQ transmissions.

FIG. 2 illustrates a method for receiving system information transmittedby a neighbor cell by using an additional HARQ process.

DETAILED DESCRIPTION

The various embodiments of the present disclosure minimize or at leastreduce the loss of packets when the mobile station tries to readneighbor cell system information. In some embodiment, the HARQ behavioron the communication link to the serving cell is adjusted for thispurpose. The disclosure relies on knowledge of the target cell's frametiming to reduce the likelihood of missing HARQ transmissions to andfrom the serving cell. The embodiments are described in terms of a 3GPPLTE system. The system information messages of the neighbor cell thatare considered include the Master Information Block (MIB) and SystemInformation Block 1 (SIB1). However, it should be clear that theteachings of the disclosure are equally applicable to other wirelesscommunication systems and other types of system information messages.

In some instances, the serving cell determines target cell information,which is provided through the system information of the target cell. Forexample, when a measurement report received from a mobile stationindicates that a target cell is a potential handover candidate, theserving cell may determine that the target cell's cell global identifieris needed

To obtain the target cell information, the serving cell sends a systeminformation acquisition order indicating or identifying the physicalcell identity (PCID) of the target cell for which additional informationis required. In some embodiments, this order includes a systeminformation acquisition time limit, wherein the mobile station isexpected to acquire the system information of the identified target cellwithin the time limit.

In some embodiments, until the expiry of the system informationacquisition time limit, the serving cell uses a lower or reduced voicecodec rate for ongoing voice calls without lowering the resourceallocation. This reduces the size of the voice packets that aretransmitted and therefore minimizes the number of HARQ retransmissionsneeded to successfully receive a voice packet. This in turn reduces thenumber of lost voice packets due to the mobile station attempting toreceive system information messages from the target cell since the sameradio resources originally used for a larger packet is now used for asmaller coded voice packet. For example, the serving network can useonly ½ rate AMR voice during this period while the resource allocationcontinues to be based on full rate AMR voice.

This technique can be applied to both uplink and downlink voice packets,i.e., both the base station and the mobile station operate at a reduceddata rate. Furthermore, this technique can be applied to obtain othersystem information as well. For example, such other system informationmay include system information blocks 2-8 defined according to the LTERelease 8 (Rel-8) specification in 3GPP TS 36.331.

Although the description above indicates that the serving cell triggersthe acquisition of the neighbor cell system information, it should beclear that the mobile station can also autonomously trigger theacquisition of the system information.

According to a second embodiment, it is assumed that the serving cellknows the frame timing difference between its transmission and that ofthe target cell. This information can be provided by the mobile stationin a measurement report, or the information may be provided by thetarget cell by using signaling mechanisms available in the network.Alternatively, the source cell and the target cell may be framesynchronized implying a frame timing difference of zero between theserving cell and the target cell. In LTE systems, since MIBtransmissions occur in sub-frame 0 and SIB1 transmissions occur insub-frame 5, if the serving cell knows the frame timing differencebetween itself and the target cell, it can determine when the mobilestation is likely to tune to the target cell. Consequently, based ontime overlap of HARQ transmissions with transmissions of MIB or SIB1 inthe target cell, the serving cell can determine which HARQ transmissionscould be missed by the mobile station.

The sub-frames in the serving cell that overlap the transmissions of MIBor SIB1 of the target cell are referred to as “unavailability periods”.According to this embodiment, the following technique is used tominimize missed transmissions:

The serving eNB sends a system information acquisition order indicatingthe PCID of the target cell. This order includes a system informationacquisition time limit. The mobile station is expected to acquire thesystem information within this time limit.

Within the system information acquisition time limit a“pre-unavailability time window” and a “post-unavailability time window”are defined around each unavailability period. The pre-unavailabilitytime window consists of n1 sub-frames before the start of theunavailability period and the post-unavailability time window consistsof n2 sub-frames after the end of the unavailability period.

The mobile station monitors for physical downlink control channel(PDCCH) in the pre- and post-unavailability time windows.

If the timing of a planned HARQ transmission is such that it occursduring the unavailability period, then it is instead transmitted duringthe pre- or post-unavailability time windows. Note that this principlecan be applied to both downlink transmissions and uplink transmissions.As an enhancement, instead of replacing the planned transmission, anadditional transmission can be performed in the pre or postunavailability time windows.

HARQ Ack/Nack behavior: To allow for easier implementation, the feedbackto a transmission in the pre- or post-unavailability period could beassumed to be a Nack. In other words, it is not necessary to assignAck/Nack resources for the shifted transmission. As an alternative, theAck/Nack resources corresponding to the originally planned transmissioncould be used. As another alternative, new Ack/Nack resources can beassigned for the shifted transmission.

The mobile station attempts to acquire the necessary system informationfrom the target cell during the unavailability periods. If successfullyacquired, the mobile station may report a part or all of the requiredacquired system information to the serving cell. Alternatively, themobile station can simply use the acquired system information forinternal processing purposes. For example, upon acquiring the CSGidentifier in SIB1, the mobile station may determine that it is notallowed to access the target cell, and therefore not report theinformation to the serving cell.

According to a third embodiment, it is assumed that the serving cellknows the frame timing difference between its transmission and that ofthe target cell. As in the second embodiment, this information can beprovided by the mobile station in a measurement report, or it may beprovided by the target cell by using signaling mechanisms available inthe network. Alternatively, the source cell and the target cell may beframe synchronized implying a frame timing difference of zero. Thefollowing steps are performed:

The serving cell sends a system information acquisition order indicatingthe PCID of the target cell. This order includes a system informationacquisition time limit. The mobile station is expected to acquire thesystem information within this time limit.

The mobile station is assigned an additional HARQ process withcorresponding resources for transmission of packets and transmission ofAck/Nacks. This assignment of the additional HARQ process could beincluded in the system information acquisition order. Note additionalHARQ process could be assigned for uplink transmissions and/or downlinktransmissions.

The additional HARQ process is used only if a planned HARQ transmissioncoincides with an unavailability period. Specifically, if a planned HARQtransmission coincides with an unavailability period, the nexttransmission opportunity on the additional HARQ process is used instead.Alternatively, if a planned HARQ transmission coincides with anunavailability period, then a transmission opportunity on the additionalHARQ process that occurs before the planned HARQ transmission can beused.

The additional HARQ process is suspended at the end of the systeminformation acquisition time limit.

As in the second embodiment, the mobile station attempts to acquire thenecessary system information from the target cell by receiving thesystem information messages when they are transmitted by the targetcell. If the system information is successfully acquired, the mobilestation may report a part or all of the acquired system information tothe serving cell. Alternatively, the mobile station can simply use theacquired system information for internal processing purposes. Forexample, upon acquiring the CSG identifier in SIB1, the mobile stationmay determine that the mobile station is not allowed to access thetarget cell, and therefore the mobile station may not report theinformation to the serving cell.

In the second and third embodiments described above, it is assumed thatthe serving cell knows the difference between its frame timing and theframe timing of the target cell (mobile station reports this frametiming difference).

In the description of embodiments above, the reading of the systeminformation is triggered by an order from the serving base station. Itshould be noted that it is equally possible to have the mobile stationtrigger the reading of the system information.

According to a fourth embodiment, since the mobile station is aware ofthe MIB and SIB1 transmission occasions of the target cell, it choosesits unavailability periods based on status information related to theHARQ processes. For example, if a MIB or SIB1 transmission in the targetcell overlaps a planned HARQ transmission in the serving cell and theHARQ transmission is one of the first n transmissions of a packet, thenthe mobile station attempts to receive the MIB or SIB1 of the neighborcell. If the planned HARQ transmission is not one of the first ntransmissions of a packet, then the mobile station receives the HARQtransmission. This allows the mobile station to receive systeminformation messages when they overlap earlier transmissions of a packetwhile allowing the packet to be still received in the later HARQtransmissions.

While the present disclosure and the best modes thereof have beendescribed in a manner establishing possession and enabling those ofordinary skill to make and use the same, it will be understood andappreciated that there are equivalents to the exemplary embodimentsdisclosed herein and that modifications and variations may be madethereto without departing from the scope and spirit of the inventions,which are to be limited not by the exemplary embodiments but by theappended claims.

What is claimed is:
 1. A method in a mobile station for acquiringneighbor cell system information, the method comprising: determining, atthe mobile station, that a transmission of a packet on a wirelesscommunication link between the mobile station and a serving celloverlaps, at least in part, a time window in which a neighbor cellperforms a transmission of a system information message the transmissionof the packet occurring on at least one of n occasions, n is greaterthan or equal to two; receiving, at the mobile station, the systeminformation message from the neighbor cell provided that thetransmission of the packet does not occur on a last m of the noccasions, m is greater than zero and less than n.
 2. The methodaccording to claim 1 further comprising: determining that thetransmission of the packet is one of the last m of the n occasions ofthe packet transmission, and receiving the transmission of the packet onthe wireless communication link between the mobile station and theserving cell when the transmission of the packet is one of the last m ofthe n occasions of the packet transmission.